Block copolymers have been employed in adhesive compositions for many years, primarily because of their high cohesive strengths and their ability to phase separate and form physical associations which act as crosslinks which are normally formed by chemical vulcanization. Block copolymers such as those described in U.S. Pat. No. 3,239,478 are either linear or multiarm polymers including radial or star styrene-butadiene or styrene-isoprene block copolymers. These polymers generally have high cohesive strength and can be readily tackified to yield good adhesive properties.
Diblocks have been used to increase tack (Korpman U.S. Pat. No. 4,080,349) and die cuttability (Miller U.S. Pat. No. 3,519,585) for some time. The diblock weight average molecular weight was equal to or greater than 50 percent of the weight average molecular weight of the triblock copolymer. Many of the early triblock copolymers contained 20 percent diblock because the coupling agents used were only 80 percent efficient. Now with the use of low molecular weight diblocks and the ability to put unsaturated diblocks with saturated triblocks, a significant reduction in viscosity may be obtained.
When one hydrogenates a diblock or a triblock copolymer, there is an upward step change in viscosity. For instance, a blend of 70 percent of a hydrogenated diblock and 30 percent of a hydrogenated triblock copolymer does have a solution viscosity in toluene @ 25.degree. C. of about 200 cps. When you replace the hydrogenated diblock with an unhydrogenated diblock, the viscosity is reduced to less than 100 cps which allows a formulation containing the polymer blend to meet the viscosity standard described herein. This blend of 30 percent SEBS (styrene-hydrogenated butadiene-styrene) and 70 percent SI (styrene-isoprene) will still give a polymer system that will impart good weatherability to the fumigation adhesive formulation, as good as or almost as good as the formulation wherein the diblock is hydrogenated. This is a very surprising and very significant discovery since the viscosity of the formulation with an unhydrogenated diblock is much lower than the formulation with a hydrogenated diblock.
All diblocks with weight average molecular weights equal to or less than the total weight average molecular weight of the triblock copolymer can be added to the corresponding triblock to reduce viscosity. However, the addition of a low molecular weight diblock reduces the viscosity much more quickly and to a much greater extent than higher molecular weight diblocks. Also, low molecular weight unsaturated diblocks are more efficient.
Fumigation adhesives were first developed from styrene-butadiene block polymers in the late 1960's. At that time the standard formulation consisted of a partially hydrogenated rosin ester tackifying resin and KRATON.RTM. D-1101 polymer. Subsequent development took place in the early 1970's where the formulations in the marketplace changed from SBS (styrene-butadiene-styrene) to SIS (styrene-isoprene-styrene) and were based on a styrene-isoprene triblock copolymer and a C.sub.5 tackifying resin. All of these products are relatively low in solids to keep the viscosity low enough for processing. Later as hydrogenated block copolymers were introduced and products developed for fumigation were designed with a hydrogenated polymer plus hydrogenated tackifying resin to give better weatherability. These fumigation adhesives are used for bonding polyethylene to polyethylene so the polyethylene can be stretched over strawberries, etc. and fumigated with the polyethylene holding the fumigant into the ground for approximately two to three weeks.
More recently, the Southern California Air Quality Management District has changed the VOC requirements in Southern California to allow only 250 VOC units or less which in these formulations is approximately 65 weight percent solids. Consequently none of the present formulations in commercial use at the present time have as much as 65 weight percent solids and in fact do not meet the Southern California VOC requirements. Such high solids content is in conflict with the need of the user to keep the solution viscosity (in toluene @ 25.degree. C.) of the formulation at 300 cps or less.
Thus, it can be seen that there is a need for polymers which can be used to produce adhesives with a better balance of properties, e.g. to be able to retain good adhesive properties and have lower viscosities at high solids content so they can meet these stringent VOC standards and still satisfy the need for a viscosity of no more than 300 cps. As will be seen below, the present invention helps to satisfy that need.